Process oils are used for facilitating rubber manufacturing processes such as kneading, extruding, or forming, on account of their good permeability into rubber polymer structures. Process oils are also used for improving physical properties of rubber products. These process oils are required to possess adequate affinity to rubbers. Rubbers to be processed include natural rubber and synthetic rubbers, with the synthetic rubbers ranging over a variety of types and kinds. Among such rubbers, natural rubber and styrene-butadiene rubber (SBR) in particular are used in large quantities, for which process oils that are rich in aromatic hydrocarbon content and possess high affinity to rubbers are generally used.
Process oils as described above are manufactured by utilizing so-called extracts produced from extraction processes using solvents having affinity to aromatic hydrocarbons, as applied to lubricating oil fractions produced through vacuum distillation of crude petroleum, or stocks obtained after appropriate dewaxing or hydrorefining, if necessary, deasphalted oil of vacuum distillation residues.
Process oils obtained in the manner mentioned above contain 70 to 99 mass % of aromatic compounds by measuring through a column chromatography, have 20 to 50% CA by n-d-M ring analysis, and contain 5 to 25 mass % of dimethylsulfoxide (DMSO) extractable content corresponding to the content of polycyclic aromatic compounds (PCA), determined in accordance with the IP346 test method as specified by The Institute of Petroleum.
However, in recent years, the carcinogenicity of PCA is being taken more seriously. In Europe, there is a regulatory movement to control the use of oils containing 3 mass % or more of DMSO extractable content, including a toxicity labeling requirement for such products. It is therefore an urgently required task to reduce the DMSO extractable content in process oils to less than 3 mass %.
Concerning process oils containing less than 3 mass % of DMSO extractable content, Japanese Patent Publication of the Translation of International Patent Application No. 6-505524 discloses a method of manufacturing a process oil, in which a vacuum distillation residue is first treated in a deasphalting process followed by a dewaxing treatment to reduce the DMSO extractable content to less than 3 mass %.
However, the process oil obtained in the above-mentioned method exhibits a high aniline point as a result of reducing the DMSO extractable content. The aniline point is a measure of aromatic hydrocarbon content in the oil. The aniline point becomes higher as the aromatic hydrocarbon content in the oil declines. A process oil having a reduced aromatic hydrocarbon content has a reduced affinity toward rubbers and therefore has a reduced degree of a quality necessary for process oils, i.e. reduced permeability into rubber polymers, thereby making it difficult to impart satisfactory physical properties to the finished rubber products.
Also in Japanese Patent Publication of the Translation of International Patent Application No. 7-501346, there are disclosed a non-carcinogenic bright stock extract and/or a deasphalted oil as well as their refining processes, wherein it is proposed to use a process parameter that correlates to the mutagenicity index (MI) as an indicator of the refining degree to reduce the MI to less than one. In the above publication, an oil obtained by deasphalting a residual fraction in a vacuum distillation column, or an oil having a reduced content of aromatic compounds obtained by an extraction of the deasphalted oil or an oil obtained by dewaxing of such an extracted oil are employed. It is however estimated that those oils may contain 3 mass % or more of DMSO extractable content. The above patent publication does not mention about the correlation between the MI and the DMSO extractable content of such deasphalted oil.
As a solution for the above-mentioned problems, the present inventors have proposed in Japanese Patent Application No. 2001-108354 a manufacturing method of a process oil having high permeability to rubber polymers and a DMSO extractable content of less than 3 mass %. However, this process oil presented a problem due to its high viscosity which provided poor working efficiency during the rubber compounding process.
The present invention has been made to solve problems as described in the above. An object of the present invention is to provide a process oil having excellent permeability into rubber polymers and high safety as provided by a DMSO extractable content of less than 3 mass % as well as improved working efficiency, and a manufacturing method thereof.